In the field of cancer chemotherapy, a large number of compounds have already been put to practical use as antitumor agents. However, their activities against various types of tumors are not necessarily satisfactory, and the problem of tolerance of tumor cells to these antitumor agents complicates their use for clinical purposes see the Proceedings of the 47th General Meeting of the Japan Cancer Society, pp. 12-15 (1988)!.
Under these circumstances, the development of novel cancerocidal substances are invariably desired in the field of cancer therapy. Among others, there is a need for substances which overcome the problem of tolerance to the existing cancerocidal substances and exhibit effectiveness against such types of cancers as cannot be effectively controlled by the existing cancerocidal substances.
In view of the above-described state of the art, the present inventors screened a wide variety of microbial metabolites, found a novel compound BE-13793C having antitumor activity (12,13-dihydro-1,11-dihydroxy-5H-indolo2,3-a!pyrrolo3,4-c!carbazole-5,7( 6H)-dione), and disclosed it see Japanese Laid-Open Patent No. 20277/'91 and the Journal of Antibiotics, Vol. 44, pp. 723-728 (1991)!.
Thereafter, they created an indolopyrrolocarbazole compound having excellent antitumor activity by a chemical modification of BE-13793C, and disclosed it (see International Publication No. WO91/18003 and European Patent Laid-Open No. EP0545195 A1).
A problem to be solved by the present invention is to create a compound having more excellent antitumor activity than the indolopyrrolocarbazole-derived antitumor substances disclosed in the prior patent application (International Publication No.
WO91/18003 and European Patent Laid-Open No. EP0545195 A1).